The present invention relates to methods and systems for controlling or managing DC machines, and more particularly to methods and systems for limiting current capability of DC machines.
Electrical power steering (EPS) systems use an electric motor as an actuator to provide assist to a driver while steering a vehicle and/or to provide vehicle control. Brushed DC machines are widely used in the Electric Power Steering (EPS) industry for low-cost applications and platforms. Electric drive systems employing such machines require fewer sensors and low-cost electronic circuitry and are able to deliver good performance through the entire operating space.
Brushed DC machines have a mechanical commutator which is connected to windings through brushes and slip rings. When current flows through the windings, this results in a current and position dependent voltage drop, commonly referred to as the brush voltage drop, which is highly nonlinear in nature. Due to the sizing of machines for low-cost applications, the machine is often operated near its capability limit. During motor control operation, the operating conditions can vary widely, and if the torque and current commands are not limited precisely, the machine invariably may cause actuator saturation (i.e., commanded voltages exceeding the bus voltage limit), resulting in undesirable performance in terms of sub-optimal command tracking, torque ripple and audible noise.